Overview
Titanium Grade 2 is the workhorse commercially-pure titanium — unalloyed, single-phase alpha, controlled by interstitial chemistry (oxygen, iron, nitrogen, carbon, hydrogen). It’s the cheapest titanium grade and the default specification for everything that needs Ti’s corrosion resistance but doesn’t need the strength of Ti-6Al-4V.
Grade 2 hits a useful sweet spot: 40 ksi yield minimum (ASTM B265), 20% elongation, excellent weldability, and the seawater corrosion resistance Ti is famous for. Compare against alternatives:
- 316L stainless is cheaper (~$5/lb vs $15/lb) but pits in hot chloride brines that Ti shrugs off
- Alloy 400 (Monel) handles HF and reducing acids better but costs roughly the same with worse oxidizing-acid performance
- Grade 5 Ti-6Al-4V has 2.5× the strength but is harder to weld, harder to form, and more expensive (~$25/lb)
- Grade 7 Ti (Pd-modified) handles hot reducing acid better than Grade 2 at significant cost premium
The selection logic is simple: pick Grade 2 when you need Ti’s corrosion resistance and Grade 2’s strength is enough. The vast majority of chemical-processing and marine titanium applications fall here.
Why titanium is expensive (and why it’s still worth it)
Titanium ore (rutile/ilmenite) is abundant — Ti is the 9th most common element in the Earth’s crust. The cost driver is the Kroll process: TiCl₄ reduction with magnesium under argon at 800–900°C, followed by vacuum distillation, sponge production, and consumable- electrode arc remelting (single, double, or triple VAR). Each step adds cost, and oxygen contamination at any stage is permanent — you can’t refine O₂ out of Ti once it’s dissolved.
The result is a metal that costs ~$15/lb for CP Grade 2 plate in commercial quantities versus ~$0.50/lb for plain carbon steel. The payback is service life: a Ti seawater cooler tube outlasts the plant it’s installed in. The capital cost is high; the life-cycle cost is often lower than copper-nickel or stainless alternatives.
Machining notes
Titanium machines slowly. The three rules:
- Run slow. 30–60 SFM for turning, less for milling. Ti’s low thermal conductivity (~16 W/m·K — 1/10 of aluminum) means cutting heat concentrates at the tool edge instead of being carried away in the chip. Run too fast and the tool fails from edge welding.
- Keep moving. Don’t dwell. A momentarily stationary tool work- hardens the surface and gall the material — restart and the tool takes a beating clearing the work-hardened zone.
- Flood coolant, never dry. The cooling job is real; this isn’t aluminum where MQL is fine. High-pressure through-spindle coolant gives the best tool life. Avoid chlorinated coolants — chlorine residue plus tensile stress causes SCC on finished Ti parts.
Sharp tools are critical. Polished carbide or cobalt HSS, positive rake, generous flute geometry to evacuate chips. Coated carbides (TiN/TiAlN) don’t help on Ti — the cutting temperature isn’t high enough to engage the coating’s thermal-barrier behavior, and chip adhesion can pull coatings off.
Workholding matters. Ti’s springback is high (low E, low yield); clamp pressures that work for steel deflect Ti parts. Soft jaws, distributed clamping, and minimum pressure. Aluminum-jaw soft fixturing works well.
Chip handling is a safety issue. Ti turnings and especially Ti dust are pyrophoric. Collect chips wet (water or coolant submerged), never accumulate dry chips. Have Class D dry powder fire suppression on hand — water accelerates burning Ti. A Ti chip fire is hot, fast, and not a thing you fight with a CO₂ extinguisher.
Welding considerations
CP Grade 2 welds excellently with one absolute requirement: shield everything that’s above ~400°C with inert gas until it cools. Ti absorbs oxygen, nitrogen, hydrogen, and carbon from air at welding temperatures, forming embrittling interstitial compounds. This isn’t “don’t get porosity” — this is “the entire HAZ becomes brittle if exposed to air while hot.”
Practical requirements:
- TIG with argon shielding, dedicated trail shield covering the bead for ~6 inches behind the torch until cool below 400°C
- Back purge on tube welds (argon flowing inside the joint)
- Welding chambers (glove boxes) for critical aerospace and medical work
- Cleanliness absolute — stainless wire brush dedicated only to Ti, acetone or MEK degrease, no chlorinated solvents (SCC risk)
- ERTi-2 filler for matching properties; ERTi-1 (purer, slightly softer) for severe corrosion service
The visual quality check: a properly welded Ti bead is bright silver. Straw yellow tint = light O₂ pickup, generally acceptable. Dark blue or purple = moderate contamination, suspect for critical service. Gray or white powdery = severe contamination, reject the weld and grind out completely before rewelding.
Preheat is not required and is generally counter-productive (longer time at risk-of-contamination temperature). PWHT is not required for CP Ti.
Corrosion behavior
Ti’s corrosion resistance comes from a thin (2–10 nm) TiO₂ passive film that forms instantly in any oxidizing environment, including air. The film is electrically insulating, chemically inert in most aqueous environments, and self-healing — scratch through it and it reforms in milliseconds.
Where Ti excels:
- Seawater — essentially immune at ambient temperature, immune to pitting up to ~80°C
- Oxidizing acids — nitric, chromic, persulfate — passive film thrives
- Wet chlorine and chlorine dioxide — pulp bleach plant standard
- Chloride brines at moderate temperature — process cooling water, swimming pool chemistry, food washdown
- Alkaline solutions — sodium hydroxide service at moderate temp
Where Ti fails:
- Hydrofluoric acid — rapidly attacked at any concentration. Even trace HF in other acids accelerates attack.
- Hot reducing acids — concentrated HCl, H₂SO₄, H₃PO₄ above ~70°C
- Dry chlorine and high-pressure pure O₂ — fire and SCC risk
- Methanol — SCC documented at low chloride levels with applied stress. This surprises designers used to Ti’s “chloride immune” reputation; pure methanol is genuinely bad for stressed Ti
- Crevice corrosion at >80°C in chloride brines — upgrade to Grade 7 (Pd-stabilized) or Grade 12 (Mo/Ni-modified)
- Hot concentrated alkalis — service possible but with attack rate; Ti is not the right material for hot caustic above 100°C
Galling: the structural Achilles heel
Titanium galls violently against itself and against most metals under sliding contact. The TiO₂ passive film is thin; once breached by sliding contact, fresh Ti-on-Ti or Ti-on-steel contact welds at the asperity scale and tears chunks of metal out.
Design rules:
- Never run Ti-on-Ti sliding contact without surface treatment or a third bearing material
- Use surface nitriding (TiN), anodizing (decorative only, doesn’t fix galling), or hard plating (electroless nickel, hard chrome) for wear surfaces
- For bushings and bearings, use bronze, DU (PTFE-impregnated bronze), or polymer journal materials — not steel-on-Ti
- Threaded fasteners: use thread lubricant (silver-plated, MoS₂, or PTFE-based) for Ti-on-Ti threading
This is the single most common Ti design failure: someone reads “Ti is corrosion-resistant and strong, let’s make this rotating shaft out of it” and discovers the rotating Ti shaft seizes against the Ti bushing the moment the lubricant migrates away.
Applications by industry
- Chemical processing — vessels, piping, heat exchangers, valves, pumps for chloride, wet chlorine, and oxidizing acid service. Grade 2 is the workhorse; Grade 7 for hot reducing service.
- Marine — fittings, fasteners, propeller shaft sleeves, sonar housings, dive watch cases, submarine hardware. Above-water fittings use Grade 2; structural marine uses Grade 5.
- Desalination — heat exchanger tubes (the dominant material for MSF and MED evaporator tubes worldwide), pump impellers, valves.
- Pulp and paper — bleach plant equipment for chlorine dioxide and hypochlorite service. Carbon steel and stainless don’t last.
- Power generation — seawater-cooled condenser tubes for coastal power plants. Titanium tube bundles last 30+ years against 8–15 for copper-nickel alternatives.
- Medical — dental implants (osseointegration is excellent), surgical instruments. Load-bearing orthopedic implants use Grade 23 ELI for fatigue toughness.
- Architectural — coastal cladding (Bilbao Guggenheim, though that was Grade 1), salt-air structural fittings.
- Aerospace (non-structural) — ducting, fluid lines, brackets in corrosive zones. Structural aerospace uses Grade 5 / Grade 23.
Failure modes worth designing around
Galling is the most common Ti design failure — see the dedicated section above. Surface-treat wear surfaces or use dissimilar bearing materials.
Hydrogen embrittlement from cathodic over-protection, acid pickling residue, or galvanic coupling to active metals. Ti picks up hydrogen slowly at ambient and faster above 80°C. ASTM B265 limits H to 0.015% in supplied material; in-service hydrogen pickup is the risk. Symptoms: brittle, low-strain fracture at unexpected loads.
Alpha case from high-temperature air exposure during heat treatment or hot forming. Oxygen diffuses into surface, forming a brittle hard layer that initiates fatigue cracks. Either heat-treat under vacuum/argon, or grind/pickle the alpha case off afterward.
Methanol SCC — surprising and well-documented. Pure methanol at ambient temperature under sustained tensile stress cracks Ti. Trace water inhibits the cracking; trace chloride accelerates it. Avoid pure methanol service entirely.
Crevice corrosion in hot chloride brines above ~80°C at gasket faces and lap joints. Open seawater immersion is fine; crevices in hot brine are not. Upgrade to Grade 7 (0.15% Pd) for that service.
Fire/ignition in dry chlorine and high-pressure O₂. Not common service, but Ti is specifically excluded from these by code in most applications. Mistakes here are dramatic.
Pyrophoric machining hazard during chip handling. Wet chip collection, Class D fire suppression, no dry chip conveyors.